This invention relates to a video format signal recording and reproducing system, and more particularly to a video format signal recording and reproducing system using a recording medium such as a video disk.
A video format signal, being similar in arrangement to an ordinary television signal, includes a series of field signals each having a data signal part bearing video data, etc., horizontal and vertical synchronizing signal parts, and horizontal and vertical blanking parts. In general, two adjacent field signals form a frame signal.
A system is well known in the art, in which such a video format signal, after being subjected to signal processing such as FM modulation, is recorded on a recording medium such as a video disk, and in which the video format signal thus recorded is reproduced at a desired place when required.
In such a recording and reproducing system, a method of inserting audio data in the data signal part through time base compression is employed so that, while a still image is being reproduced, oral explanation may be made.
A typical example of this method will be described with reference to a video disk. In a video disk, each circular recording track is regarded as a period corresponding to one TV frame. Audio data is recorded in the first through (N-1)-th ones of the N frames, while video data is recorded in the N-th frame. These N frames are handled as one group.
In reproducing the video disk, the audio data recorded in the first through (N-1)-th frames is read and is temporarily stored in a memory, and the recording track corresponding to the N-th frame is repeatedly subjected to reproduction to obtain the still image, while the audio data is read out of the memory and reproduced after being subjected to time base expansion to real time.
The frequency band of a video signal, which can be recorded on a video disk, ranges from a DC component to a 4 MHz component. If the frequency band of the audio signal extends up to 15 KHz, then the frequency band of the video signal is about 260 times that of the audio signal. Therefore, for one TV frame period (about 1/30 second), an audio signal of about 8.5 seconds can be recorded after being subjected to time base compression. In reproducing still image data, 15 to 30 seconds are sufficient to explain the image data orally. For the abovedescribed reason, a series of audio data are recorded in two to four TV frames, after being subjected to time base compression, and the still image data is recorded in another TV frame.
In reproducing the video disk in which data have been recorded according to the above-described system, since almost all of the frames are for audio data and only the remaining frames are for the video data, if the frames containing audio data signals are displayed on the cathode ray tube, they appear as noise against the necessary video signals. The appearance of the noise may be eliminated by clamping the image to a color (black, white or gray) at a certain level; however, in this case, the image is erased momentarily, as a result of which the so-called "flickering phenomenon" is caused, which is uncomfortable for the person observing the image.